 "
   {:arglists '([relations+])}
   [& relations]
-  (raw/union$ relations {}))
+  (raw/union$ (filter identity relations) {}))
 
 (defn intersection
   "Performs an intersection on all relations provided and returns the distinct

"
  {:arglists '([relations+])}
  [& relations]
  (raw/union$ relations {}))

(defn union-multiset
  "Performs a union on all relations provided and returns all results.

  {:arglists '([opts? relations+])}
  [& opts-relations]
  (let [[opts relations] (split-opts-relations opts-relations)]
    (raw/distinct$ (raw/union$ (filter identity relations) {}) opts)))

(defn concat
  "Concatenates all relations provided. Does not guarantee any ordering of the
